Optical interferometers of various forms (reference 1) have been used to measure the surface profiles of samples. Unmodulated interferometric systems require fringe counting and have a nonlinear distance-signal relationship. Modulated interferometric approaches such as laser heterodyne (reference 2) and wavelength multiplexed interferometry (reference 3) improve the linearity and range of the profilometer, but are complex to implement. The present invention presents a pulsed profilometer which is insensitive to sample mechanical vibrations, has a linear range of several orders of magnitude, and is simpler to implement than prior approaches.
The present invention describes a new surface profilometry system. The system relates the frequency content of the signal from a scanning autocorrelator to the surface variations of a sample placed in one of the arms of the autocorrelator. This profilometry system solves the following problems of the previous approaches:
1. Conventional interferometers have a linear range limited by the wavelength of light. The system of the present invention has a much larger linear range, typically from angstroms to microns, which is determined by the laser pulsewidth. PA1 2. Wide dynamic range interferometers, like those suggested by Williams and Wickramasinghe, require multiple laser sources or multiple wavelength lasers. The system according to the present invention uses only a single pulsed laser source. PA1 3. Conventional profilometers are sensitive to the sample vibrations. The system according to the present invention is insensitive to sample vibrations that are slow compared to the scanning rate of the autocorrelator.